1. Field
The embodiments discussed herein are directed to an audio decoding apparatus and an audio decoding method that include audio signals having channels of a number different from the number of channels of original audio signals.
2. Description of the Related Art
In recent years, digitalization of broadcasting including television broadcasting and radio broadcasting increased. For example, digital broadcast services including terrestrial digital television broadcasting, broadcasting satellite/communication satellite (BS/CS) digital broadcasting, and terrestrial digital audio broadcasting are provided in Japan. Such digital broadcasting adopts, for example, Moving Picture Experts Group phase 2 Advanced Audio Coding (MPEG-2 AAC) scheme capable of supporting multiple channels as a method of encoding audio signals. Accordingly, the digital broadcasting delivers many pieces of content including 5.1-channel audio outputs having a presence more excellent than that of stereos in related art. The 5.1-channel is hereinafter denoted by 5.1-ch. Similarly, a 3.1-channel and a 7.1-channel are hereinafter denoted by 3.1-ch and 7.1-ch, respectively.
However, audio decoding apparatuses that receive digital broadcasts to reproduce audio signals include many apparatuses that do not support decoding and reproduction of 5.1-ch audio signals. Consequently, down-mixing techniques are required to include audio signals, such as stereo audio signals, having channels of a number that is smaller than the number of channels of original multi-channel audio signals from the multi-channel audio signals, such as 5.1-channel audio signals.
Such down-mixing techniques include a technique to perform a down-mixing process on frequency-domain audio signals and convert the frequency-domain audio signals subjected to the down-mixing process into time-domain audio signals.
For example, refer to Japanese Laid-open Patent Publication No. 1997-252254, Japanese Laid-open Patent Publication No. 2000-29498, and Japanese Laid-open Patent Publication No. 2007-531913.
In contrast, in the MPEG-2 AAC scheme mentioned above, Modified Discrete Cosine Transform (MDCT) is used to encode audio signals and time-domain audio signals are converted into frequency spectra. Audio encoding apparatuses adopting the MPEG-2 AAC scheme vary the length of a window, which is the processing unit in the MDCT, depending on the characteristics of the audio signals when MDCT processing is performed on the audio signals. For example, a typical audio encoding apparatus performs the MDCT processing on audio signals including a stationary sound by using a window including 2,048 sample points of the audio signal. In contrast, the audio encoding apparatus performs the MDCT processing on audio signals including a sound, such as an attack sound, which varies in a short time by using a window including 256 sample points of the audio signal. Accordingly, different lengths of windows may be used in different channels in the audio signals encoded by the audio encoding apparatus.
In such a case, a typical audio decoding apparatus adopting the down-mixing technique in the related art described above cannot directly perform the down-mixing process on frequency-domain audio signals because the frequency-domain audio signals in different channels are calculated by using different time lengths. In addition, the audio decoding apparatus in the related art performs Inverse Modified Discrete Cosine Transform on the frequency-domain audio signals in each channel before the down-mixing process is performed to convert the frequency-domain audio signals into time-domain audio signals. The Inverse Modified Discrete Cosine Transform is hereinafter denoted by IMDCT. Furthermore, in the audio decoding apparatus in the related art, it may be necessary to perform the MDCT processing again on the time-domain audio signals in all the channels by using a common window. As described above, it may be necessary to perform the MDCT processing and the IMDCT processing on the audio signals in the respective channels in order to perform the down-mixing process in the audio decoding apparatus in the related art, so that an enormous amount of calculation is required.